Spring is in many respects a celebration of color, as plants and animals emerge from a very blah-colored winter dormancy. We have been barraged by a profusion of pinks, reds, oranges, yellows, golds, browns, chestnuts in the past couple of weeks as the spring migrants have passed through here. These bright colors are all products of pigments synthesized or ingested and metabolized by their exhibitors. You’ll notice I left some colors out.
The blues: Blue color in animals is produced by structural reflections of incoming light, not a pigment. As a result, the true coloration on an animal often changes as their position changes relative to the observer.
The greens: Green color is a mixture of blue and yellow, so green is a mixture of pigment and structural reflection.
Here’s how it works. The underlying color of a blue bird is black (or brown) and is produced by melanin pigment. Feathers that appear blue have air spaces in their pigment-containing cells that reflect short wavelengths (blue light) but transmit longer wavelengths. Thus red and yellow (longer) light wavelengths are absorbed by the melanin pigment and blue light is amplified and reflected back to the observer’s eye. The structural organization of the air spaces determines the intensity and hue of the “blue-ness”.
A similar process of light scattering and absorption explains blue colors in insects, like dragonflies. In some species, the outer cuticle is transparent, and the scattering of blue light and absorption of longer wavelengths occurs in the layer just beneath the cuticle. This is the basis for the blue color of the male Eastern Pondhawk.
In other species, the light scattering occurs in a waxy layer above the cuticle, and can be removed by dissolving the wax. The animal then becomes the color of the underlying pigment.
There is no doubt that blue coloration is striking and noticeable in animals, and it seems to be used to call attention to some aspects of the animal’s anatomy. What could be more striking than seeing an animal advertise this?